 5S ribosomal RNA

5S ribosomal RNA is a small, roughly 120-nucleotide ribosomal RNA of the large subunit of cytoplasmic ribosomes. Its folded RNA contributes to the central protuberance, together with ribosomal proteins and parts of the larger ribosomal RNA. In bacterial ribosomes, it associates with the 50S subunit. The name refers to a sedimentation coefficient, not a count of five bases. Its conserved fold contains five helices and five loops.^[ref]

The role of a component can be tested by changing its physical independence. In a 2020 experiment, researchers joined a rearranged 5S sequence to the much larger 23S ribosomal RNA and removed the ordinary free 5S copies from Escherichia coli. Some engineered cells survived and their properly assembled ribosomes remained active in translation. However, defective large subunits accumulated. Cryo-electron microscopy showed malformed regions of the catalytic center in these misassembled particles.^[ref] Survival therefore distinguishes the need for the 5S sequence from the need to keep it as a separate RNA; it does not establish that assembly is unaffected.

In eukaryotes, 5S ribosomal RNA is transcribed by RNA polymerase III, unlike the major precursor transcribed by RNA polymerase I. The 1974 polymerase study identified the polymerase-III association through distinct inhibition responses of RNA synthesis in isolated nuclei and separated enzyme preparations.^[ref]